How to build a Pareto chart for quality defects
The Pareto chart came out of quality control, and it is still where it shines. Count defects by type, sort them, add a cumulative line, and the inspection list writes itself.
· 5 min read
Example defect counts from one production run
| Defect | Occurrences |
|---|---|
| Surface scratch | 420 |
| Misalignment | 260 |
| Weld failure | 180 |
| Paint defect | 96 |
| Loose fastener | 54 |
| Labeling error | 22 |
Illustrative counts. Swap in your own inspection log for a real chart.
The four ingredients of a correct Pareto chart
A Pareto chart is not just a sorted bar chart. It needs all four elements to support a decision.
- Bars sorted from largest to smallest count
- A secondary axis with the cumulative percentage
- A visible threshold line, usually 80%
- Consistent categories across runs so charts stay comparable
Counts or cost?
Counting occurrences answers "what happens most often". Weighting by rework cost or scrap value answers "what hurts most". They often produce different shortlists.
A rare weld failure can outrank a frequent scratch once cost is included. Run both charts when the values differ widely.
Close the loop
After a corrective action, chart the next production run with the same categories. If the top bar shrank and the curve flattened, the fix worked. If a new category rose to the top, that is your next cycle.
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Example defect counts from one production run
- Total
- 1,032
- Categories
- 6
- Vital few
- 3
- Top 20% share
- 65.9%
Priority #1
Surface scratch
40.7% of total · 420
| # | Category | Value | Share | Cumulative |
|---|---|---|---|---|
| 1 | Surface scratch Vital few | 420 | 40.7% | 40.7% |
| 2 | Misalignment Vital few | 260 | 25.19% | 65.89% |
| 3 | Weld failure Vital few | 180 | 17.44% | 83.33% |
| 4 | Paint defect | 96 | 9.3% | 92.64% |
| 5 | Loose fastener | 54 | 5.23% | 97.87% |
| 6 | Labeling error | 22 | 2.13% | 100% |
3 of 6 categories account for 83.3% of total impact.
Vital few: Surface scratch, Misalignment, Weld failure.
Surface scratch is the largest contributor at 40.7%.
Surface scratch represents 420 of a total of 1,032.
Moderate concentration: the top 20% of categories hold 65.9%.
There is a leading group, but the long tail still matters.
3 categories share the remaining 16.7%.
These are the trivial many under the Pareto principle.
Review Surface scratch first.
It is the single largest contributor at 40.7% of total impact.
Review opportunities across the wider process.
Impact is spread out — the top 20% only accounts for 65.9%.
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Key takeaways
- Sorted bars plus a cumulative line, or it is not a Pareto chart.
- Chart cost as well as counts when defects differ in severity.
- Compare consecutive runs with identical categories.